The monitoring group also reported that over the past few days coalition aircraft have been dropping leaflets over the northern suburbs of the city, asking civilians for the first time to leave the areas controlled by ISIS.

"This is the time you have been waiting for. It is time to leave Raqqa," a report on the Raqqa is Being Slaughtered Silently website says.

However, it appears the initial operation will be limited in scope.

Idriss Nassan, a spokesman for the Kurds in Kobani, said there had already been some clashes in the villages around Ain al Issa, 30 miles north of Raqqa, but he did not believe the offensive from the north and east was aimed at the city of Raqqa itself.

The U.S.-led coalition is providing assistance and advice to the Syrian Democratic Forces in the battle, coalition spokesman Col. Steve Warren said.

There are personnel providing "advice and assistance" but there are not forces on the front line, he said.

"We are in their off centers and headquarters providing advice," he added, and not "exchanging fire" with ISIS.

Idriss Nassan, a spokesman for the Kurds in Kobani, Syria, said there had already been some clashes in the villages around Ain al Issa, 30 miles north of the northern Syrian city, which has been the terror group's de facto capital since it was taken in 2013.

State of emergency?

Earlier this month U.S. officials confirmed that the jihadi group had declared a state of emergency in Raqqa as Kurdish and Syrian rebel troops consolidated gains around the city.

Outside help

Russia is ready to coordinate efforts with the U.S.-led coalition and the Kurdish forces in Syria, Russian Foreign Minister Sergey Lavrov said Tuesday. "I cannot say how real reports are that such actions have already begun, but we are ready for such coordination," Lavrov said in Uzbekistan.

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Warren said the coalition has "always been focused on evicting ISIS" from Raqqa. "We will continue to support the (Syrian Democratic Forces) as they conduct ground operations to further isolate the city."

He added that the coalition will continue to provide air support in the battle against ISIS.